1. Field of the Invention
The invention relates to the field of reducing the noise in a gas-flow duct and more particularly in a gas turbine engine.
2. Description of the Related Art
A gas turbine engine for an aircraft such as an airplane or a helicopter usually comprises, from upstream to downstream in the direction of flow of the gases, one or more compressor stages, a combustion chamber, one or more turbine stages and a gas exhaust device such as a nozzle.
A constant problem for the engine manufacturers is the reduction of noise, notably for the comfort of the passengers and of the inhabitants of the zones overflown by the aircraft. In particular, helicopters travel close to inhabited zones and the noise of their exhaust nozzle forms a considerable component of the total noise that they generate. The attenuation of the noise leaving through the nozzle may be obtained by the use of an acoustic attenuation covering forming the inner wall of the nozzle and hence the outer jacket of the gas path. Such a covering may for example comprise a perforated metal sheet emerging on one or more resonance cavities, each assembly of a cavity and of one or more holes forming a Helmholtz resonator. The cavities may for example have a structure of the honeycomb type or of the tubular type.
Such attenuation coverings or acoustic treatment usually allow the attenuation of sound frequencies known to be average, for example of between 0.8 and 5 kHz. The sound frequencies that are treated depend notably on the depth of the resonance cavities that are therefore dimensioned accordingly.